Aspects and embodiments of the invention are in the field of optical sensors, optical sensing methods, and applications thereof. Aspects and embodiments more particularly pertain to an imaging apparatus, methods, and applications and most particularly to a wavelength (color)- and polarization-sensitive pixel device, a color- and polarization-sensitive focal plane array comprising a plurality of said pixel devices, a color- and polarization-sensitive microbolometer apparatus, and associated methods and applications.
Polarization is a property of light wherein rays of light have different amplitudes in different directions. The polarization state of light is invisible to the unaided human eye. People can differentiate colors coming from an object but cannot distinguish the different directions of polarization. Photographers often use a polarizer with a camera to reduce glare and to improve contrast, but these are add-on components and typically are not part of the detector systems. Conventional cameras, including digital cameras, are polarization blind without these add-on components.
A conventional digital color camera uses small color filters such as red, green, and blue filters to measure color information at different pixels. In order to image the polarization state of light, a polarization camera may use a small polarizer filter with dimensions equal to the size of the pixel.
An imaging polarimeter can be used to sample the polarization signature across a scene with the recorded images quantified as Stokes vectors S, which consist of the four elements S0, S1, S2, and S3. S0 represents the intensity of an optical field; S1 and S2 denote the affinity towards 0° and 45° linear polarization respectively; and S3 expresses the difference between right and left circular polarizations. Using S, the angle of linear polarization, the degree of polarization (DOP), the degree of linear polarization (DOLP), and the degree of circular polarization (DOCP) across a scene can be derived and investigated.
                              θ          linear                =                              1            2                    ⁢                      tan                          -              1                                ⁢                                    S              2                                      S              1                                                          (        1        )                                DOP        =                                                            S                1                2                            +                              S                2                2                            +                              S                3                2                                              /                      S            0                                              (        2        )                                DOLP        =                                                            S                1                2                            +                              S                2                2                                              /                      S            0                                              (        3        )                                DOCP        =                              S            3                    /                      S            0                                              (        4        )            
Measurement of polarization provides valuable information. Such information has been used, e.g., to construct the 3D shape of an unknown object, to image a target through dust, clouds, haze, and water, and to identify malignant cancer cells from healthy tissue. Polarization cameras enable measurement of polarization states and novel imaging applications in optical sciences and engineering. Polarization imaging has important applications in material sciences, medicine and remote sensing. Using infrared wavelengths, specific applications include surveillance and night vision, with a camera capable of being attached to, e.g., a fixed object, an autonomous vehicle, or a drone.
The properties of optical filters can be described by the Mueller matrix, which is a 4×4 matrix with coefficients that are generally wavelength and angle dependent. Incoming light described by the Stokes vector S′ is transmitted through an optical filter described by a Mueller matrix M and is converted to outgoing light described by a Stokes vector S as follows:
                                          [                                                                                m                                          0                      ,                      0                                                                                                            m                                          0                      ,                      1                                                                                                            m                                          0                      ,                      2                                                                                                            m                                          0                      ,                      3                                                                                                                                        m                                          1                      ,                      0                                                                                                            m                                          1                      ,                      1                                                                                                            m                                          1                      ,                      2                                                                                                            m                                          1                      ,                      3                                                                                                                                        m                                          2                      ,                      0                                                                                                            m                                          2                      ,                      1                                                                                                            m                                          2                      ,                      2                                                                                                            m                                          2                      ,                      3                                                                                                                                        m                                          3                      ,                      0                                                                                                            m                                          3                      ,                      1                                                                                                            m                                          3                      ,                      2                                                                                                            m                                          3                      ,                      3                                                                                            ]                    ⁢                      (                                                                                S                    0                    ′                                                                                                                    S                    1                    ′                                                                                                                    S                    2                    ′                                                                                                                    S                    3                    ′                                                                        )                          =                  (                                                                      S                  0                                                                                                      S                  1                                                                                                      S                  2                                                                                                      S                  3                                                              )                                    (        5        )            
Important parameters of the optical filters are diattenuation, D, which describes the difference in maximum and minimum transmittances of orthogonal polarization states, polarizance, P, which describes the degree of polarization of the exiting light, and depolarization index, DI, which describes the deviation from an ideal depolarizer.
                    D        =                                                            T                max                            -                              T                min                                                                    T                max                            +                              T                min                                              =                                                                      m                                      0                    ,                    1                                    2                                +                                  m                                      0                    ,                    2                                    2                                +                                  m                                      0                    ,                    3                                    2                                                                    m                              0                ,                0                                                                        (        6        )                                P        =                                                            m                                  1                  ,                  0                                2                            +                              m                                  2                  ,                  0                                2                            +                              m                                  3                  ,                  0                                2                                                          m                          0              ,              0                                                          (        7        )                                DI        =                                                                              ∑                                      i                    ,                                          j                      =                      0                                                        3                                ⁢                                                                  ⁢                                  m                                      i                    ,                    j                                    2                                            -                              m                                  0                  ,                  0                                2                                                                        3                        ⁢                          m                              0                ,                0                                                                        (        8        )            
An ideal polarizer preserves one eigenpolarization and completely eliminates the other eigenpolarization over two or more bands of wavelengths. For example, an achromatic polarizer covers both the near infrared (0.75 to 1.4 micron) and short wavelength infrared (1.4 to 3 micron). The ideal magnitudes of diattenuation and polarizance are one (1), and the depolarization index is zero (0). The transmittance or reflectance of the desired eigenpolarization is 100%, and those of the other eigenpolarization are 0%. In addition, for polarization imaging, the response in transmission and diattenuation should be constant at wide angle ranges because the incoming rays of light through an imaging lens can have a wide range of angles depending on the numerical aperture (N.A.) or f-number (f/#) of the lens.
In order to achieve the ideal response for the optical filter, i.e., broadband, wide angle, high extinction ratio, low loss etc., the optical filter is often made of multiple layers of materials. For color optical filters the materials can be multi-layer dielectric interference filters, dichroic materials, meta-materials, and/or color photo resists based on dye and/or pigment. For retarders and polarizers the materials can be liquid crystal polymers, birefringent materials, meta-materials, and/or a wire-grid in circular, linear, or elliptical configurations. For a birefringent material with index difference Δn between two eigen-axes, the phase shift Δϕ between two eigenpolarizations increases with the thickness, d, of the material relative to the wavelength λ.
As the operating wavelength increases, the thickness of the filter must also increase in order to achieve the same phase shift. For normal dispersion of materials, both n and Δn generally decrease with increasing λ. For mid infrared wavelengths (3-8 micron), a single layer of birefringent material can be several microns thick. A broadband and wide angle design often requires multiple layers of materials. This is also true for interference filters, where increasing the thickness of the filter can improve the contrast of the filter. An ultra-broadband infrared optical filter operating from 0.5 to 11.5 microns can have a thickness of as much as 70 microns. For comparison, the size of a sensor pixel is of the order of 1 to 20 microns. For conventional imaging using a Bayer filter, the ratio of optical filter thickness to pixel size is less than one (<1), and a large percentage, approximately 70% to 90%, of the incident light at different angles is collected by the sensor after passing through the optical filter. For infrared wavelengths the same ratio can be greater than one (>1) and only a small percentage, approximately 20% to 50%, of the incident light is collected by the sensor because of the thickness of the IR filter. Light that is incident at large angles may be collected by adjacent pixels, leading to cross talk.
In view of the problems and challenges presented by the use of ‘thick’ optical filters as described hereinabove and appreciated by those skilled in the art, the inventor has recognized the advantages and benefits of providing an optical structure incorporating such thick filters that mitigates or eliminates the disadvantages of narrow acceptance angles, low light collection, low contrast, narrow bandwidth, crosstalk, high expense, and difficulty of fabrication, which said mitigation or elimination is realized by the embodied invention.